The Proterozoic Eon (2500–543 million years ago) saw Earth's oceans as moderately oxic at the surface and sulfidic at depth, affecting trace metal availability and limiting eukaryotic algae distribution. Recent evidence suggests "intermediate" oceans, with surface oxygen and deep anoxic/sulfidic conditions, persisted for over 1000 million years. Sulfidic deep oceans, indicated by banded iron formations (BIFs) and sulfur isotope data, may have been common during this time, influencing bioinorganic cycles and eukaryotic evolution. Sulfidic conditions in the mid-Proterozoic, supported by sulfur isotope fractionation and sedimentary records, suggest limited availability of trace metals like Fe and Mo, which are essential for nitrogen fixation and other biological processes. This scarcity would have restricted nitrogen availability, impacting primary productivity and eukaryotic evolution. The scarcity of these metals in sulfidic oceans may explain the limited diversity of eukaryotic algae and the delayed diversification of complex eukaryotic life forms. The development of sulfidic oceans led to a period of nitrogen stress for the biosphere, as nitrogen fixation was limited by the scarcity of essential metals. This stress is reflected in the stable carbon isotope ratios (δ¹³C) during the mid-Proterozoic, indicating reduced primary productivity and limited nutrient availability. The rise of oxygenation and the subsequent availability of trace metals allowed for increased nitrogen cycling and eukaryotic diversification. The interplay between ocean redox conditions and bioinorganic cycles provides a bridge between environmental and biological evolution. The scarcity of trace metals in sulfidic oceans limited nitrogen availability, affecting eukaryotic evolution and the development of more complex life forms. The transition to more oxidized oceans and the availability of essential metals facilitated the diversification of eukaryotic life and the expansion of nitrogen cycling processes. This study highlights the importance of bioinorganic chemistry in understanding the coevolution of life and environment. The findings suggest that the mid-Proterozoic oceans, characterized by sulfidic conditions and limited trace metal availability, played a crucial role in shaping the evolution of eukaryotic life and the development of more complex biological systems. Future research should focus on integrating geochemical and biological data to further elucidate the relationship between ocean redox conditions and the evolution of life on Earth.The Proterozoic Eon (2500–543 million years ago) saw Earth's oceans as moderately oxic at the surface and sulfidic at depth, affecting trace metal availability and limiting eukaryotic algae distribution. Recent evidence suggests "intermediate" oceans, with surface oxygen and deep anoxic/sulfidic conditions, persisted for over 1000 million years. Sulfidic deep oceans, indicated by banded iron formations (BIFs) and sulfur isotope data, may have been common during this time, influencing bioinorganic cycles and eukaryotic evolution. Sulfidic conditions in the mid-Proterozoic, supported by sulfur isotope fractionation and sedimentary records, suggest limited availability of trace metals like Fe and Mo, which are essential for nitrogen fixation and other biological processes. This scarcity would have restricted nitrogen availability, impacting primary productivity and eukaryotic evolution. The scarcity of these metals in sulfidic oceans may explain the limited diversity of eukaryotic algae and the delayed diversification of complex eukaryotic life forms. The development of sulfidic oceans led to a period of nitrogen stress for the biosphere, as nitrogen fixation was limited by the scarcity of essential metals. This stress is reflected in the stable carbon isotope ratios (δ¹³C) during the mid-Proterozoic, indicating reduced primary productivity and limited nutrient availability. The rise of oxygenation and the subsequent availability of trace metals allowed for increased nitrogen cycling and eukaryotic diversification. The interplay between ocean redox conditions and bioinorganic cycles provides a bridge between environmental and biological evolution. The scarcity of trace metals in sulfidic oceans limited nitrogen availability, affecting eukaryotic evolution and the development of more complex life forms. The transition to more oxidized oceans and the availability of essential metals facilitated the diversification of eukaryotic life and the expansion of nitrogen cycling processes. This study highlights the importance of bioinorganic chemistry in understanding the coevolution of life and environment. The findings suggest that the mid-Proterozoic oceans, characterized by sulfidic conditions and limited trace metal availability, played a crucial role in shaping the evolution of eukaryotic life and the development of more complex biological systems. Future research should focus on integrating geochemical and biological data to further elucidate the relationship between ocean redox conditions and the evolution of life on Earth.